Study finds coralline algae are threatened by the dual stressors of ocean acidification and warming

When you think of algae, you may think of the slick, green organisms that coat surfaces of lakes and cling to rocky shorelines. But not all algae are slimy and buoyant: Coralline algae are hard, look similar to coral and form pink and purple mats on the seafloor. 

They're also particularly susceptible to ocean acidification and warming, according to a recent California Sea Grant-funded study published in the Journal of Phycology. Researchers worked in a laboratory to examine how the temperate articulated coralline alga Calliarthron tuberculosum responded to warmer temperatures and lower seawater pH (indicating ocean acidification). They found the algae was negatively affected by both ocean acidification and warming, and the combined threat of ocean warming and acidification made it more difficult for the algae to grow and calcify — grow their skeletons — than acidification or warming alone.

Emily Donham, postdoctoral researcher at the University of California, Santa Barbara, said she and her team chose to focus on this species of coralline algae because science is clear that coralline algae as a whole is particularly susceptible to the threat of ocean acidification. But until now, not many studies have looked at the interactive effects of ocean acidification and warming — especially in coralline algae in temperate regions. The laboratory setup of the study allowed scientists to look closely at how this alga responds to changing ocean conditions. Science shows that, as humans emit carbon dioxide and other greenhouse gases into the atmosphere, the ocean is warming and becoming more acidic. 

“We had this really controlled laboratory environment where we could tightly manipulate the pH and temperature that the organisms were exposed to,” she says. “We were able to expose them to normal present-day conditions — what they might experience currently out in nature — and then turn up the temperature and turn down the pH.”

Coralline algae might not be some of California’s best known marine species, but they are important — both ecologically and economically. They thrive in kelp forests, creating pink and purple patches on the seafloor that Donham compares to bubblegum.

“You can go out on the intertidal zone and see them covering tidepools,” she says. Underwater, “it’s kind of like swimming over these pink and purple mats of this calcified grass.” 

These mats serve as critical nurseries for ecologically and commercially important invertebrate species. Scott Hamilton, co-author of the study, stressed how important these coralline algae are for marine creatures in the kelp forest. 

“This coralline habitat is a really important nursery for small little crabs, snails, shellfish and sea urchins that hide in this complex 3D habitat,” he says. “If you lose some of that habitat, that could have negative consequences for species that use it, especially in their juvenile stages.” 

That habitat loss could occur as ocean waters continue to warm and ocean pH continues to decrease. Coralline algae are calcifying organisms: creatures such as coral and shellfish that use calcium carbonate to build shells and skeletons. These organisms are particularly susceptible to ocean acidification because lower ocean pH can make shell and skeleton growth difficult. 

Donham, whose research focuses broadly on how environmental change will affect kelp forest species, says the study is important in part because, in spite of their importance to the marine ecosystem, coralline algae aren’t a well-known group of organisms.

“This really just stresses the point that acidification and warming are likely to have some negative impacts on the species that live within our kelp forests and along our coasts,” Donham says. “That includes species that we may not think about much, like the coralline algae that aren’t something that you have on your dinner plate or are charismatic like the giant kelp. These species are likely to be impacted, and they play a major role in the functioning of these ecosystems.”